GB2346122A

GB2346122A - Active vehicle suspensions

Info

Publication number: GB2346122A
Application number: GB9901875A
Authority: GB
Inventors: Theodore Edmund Cooper Potter
Original assignee: MG Rover Group Ltd
Current assignee: MG Rover Group Ltd
Priority date: 1999-01-29
Filing date: 1999-01-29
Publication date: 2000-08-02
Also published as: GB9901875D0

Abstract

A pair of hydro-pneumatic suspension units 14, 16 are interconnected by a control actuator comprising a piston 44 operating between two working chambers 48, 50. If the vehicle tends to roll to one side, e.g. during cornering, a valve 56 is used to connect the larger area chamber 48 to the suspension unit on the outside of the corner and the smaller area chamber 50 to the suspension unit on the inside of the corner and the piston 44 is moved to increase the volume of the larger area chamber and to decrease the volume of the smaller area chamber. This simultaneously counteracts the tendency to roll and reduces the total fluid volume in the suspension units counteracting the associated tendency of the vehicle ride height to increase. A similar system controlling four wheels, thus the roll and pitch of the vehicle, is also disclosed (fig 2).

Description

VehicleSuspensions The present invention relates to active vehicle suspensions and is particularly concerned with roll and pitch control in such systems.

It is known to provide active suspension systems in which a hydraulic jack or actuator acts in series with the coil spring or air spring associated with each wheel and the fluid pressures in the actuators is controlled so as to control the roll and pitch of the vehicle. However it can be a problem with such systems that, because the hydraulic pressure in the actuators supports the weight of the vehicle, the power needed to control the actuators is significant. It also tends to be a problem with such systems that under roll conditions where weight is transferred from one side of the vehicle to the other, the degree of expansion in the coil or air spring on the rising side of the vehicle is greater than the degree of compression of the spring on the dropping side. This is a simple result of the characteristic of the springs. This can lead to instability of the vehicle when cornering.

It is an aim of the present invention to provide a suspension system which is less susceptible to these problems.

Accordingly the present invention provides a vehicle suspension system for controlling movement of two vehicle wheels relative to a vehicle body, the system comprising two suspension assemblies each associated with one of said wheels and each comprising a spring and a hydraulic actuator acting in series to control movement of the respective wheel relative to the body, and control means arranged to detect a condition when the wheels will tend to move in opposite directions relative to the body, and in response to cause hydraulic fluid to flow out of a first one of the hydraulic actuators and into a second one thereby to counteract the tendency to move, wherein the control means is arranged to cause more fluid to flow out of the first hydraulic assembly than to flow into the second thereby reducing the total volume of fluid in the two actuators.

The hydraulic actuators can take any suitable form such as piston and cylinder assemblies or piston and rolling diaphragm assemblies.

The condition when the wheels will tend to move in opposite directions relative to the body can be detected by a variety of means. For example lateral or longitudinal accelerometers mounted on the vehicle body, steering angle and vehicle speed sensors, or braking sensors could be used.

Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a diagrammatic representation of a vehicle suspension system according to a first embodiment of the invention, and Figure 2 is a diagrammatic representation of a vehicle suspension system according to a second embodiment of the invention.

Referring to Figure 1, in a first embodiment of the invention an active suspension system operates only on the front wheels 10, 12 of a vehicle, the rear wheels being connected to the vehicle body by a passive suspension. A hydropneumatic suspension unit 14,16 is provided to connect each of the front right and left wheels 10, 12 to the vehicle body, each unit comprising an air spring 18 and a hydraulic actuator in the form of a piston and cylinder assembly 20 connected in series. The piston 22 of the hydraulic assembly is connected to the wheel 10,12 and the cylinder 24 is mounted on the vehicle body. The cylinder 24 is hydraulically interconnected via a pipe 25 to the hydraulic chamber 26 of a gas spring sphere 28 which includes a flexible diaphragm 30 dividing the hydraulic chamber 26 from the gas spring 18.

A control actuator 40 comprises a cylinder 42 with a double ended piston 44 slidable in it. The cylinder 42 has a stepped bore with a narrower part 42a and a wider part 42b which from a shoulder 42c between them. The piston 44 has long cylindrical section 44a which is a sliding fit in the narrower part 42a of the cylinder bore, and a wider piston face 44b at one end which is a sliding fit in the wider part 42b of the cylinder bore. The cylinder is therefore divided into an actuation chamber 46 in the narrow part 42a of the cylinder, a first working chamber 48 at the opposite end of the cylinder, and a second working chamber 50 of annular cross section around the cylindrical section 44a of the piston 44.

The two working chambers 48,50 each have an inlet/outlet port 52,54 which is connected to a control valve 56. The control valve 56 has three states, a first one of which closes off the ports 52,54, a second one of which connects each of the working chambers to a respective one of the interconnecting pipes 25 of the suspension units 14,16, and a third one of which swaps over the connections of the second.

The actuation chamber 46 has an inlet/outlet port 58 connected to an actuation valve 60 which also has three states, one of which shuts off the port 58, one of which connects it to a source of hydraulic pressure in the form of a pump 62 and accumulator 64, and the third of which connects it to a drain in the form of a reservoir 66. A control unit 68 is also provided which controls the two valves 56,60 and the pump 62 so as to control the flow of fluid between the suspension units 14, 16 and the control actuator 40. In order to do this it receives signals from lateral accelerometers 70 mounted on the vehicle body and arranged to measure lateral acceleration of the vehicle, wheel speed sensors 72 associated with the an ABS system for the vehicle which allow measurement of the longitudinal acceleration of the vehicle, i. e. in the direction parallel to the direction of travel, and ride height sensors 74 which measure the height of the vehicle body above each of the wheels.

In use, when the vehicle is travelling in a straight line on a level surface and therefore not tending to roll, the piston 44 of the control actuator 40 rests at the left end of the cylinder 42 as shown in Figure 1, with the first working chamber 48 at its minimum volume and the second working chamber 50 at its maximum volume. When the control unit 68 determines from the lateral accelerometers that the vehicle is experiencing a lateral acceleration, and is therefore liable to roll by rotating about the horizontal longitudinal axis L-L, it controls the control actuator 40 as described below so as to counteract the tendency to roll.

Assuming that the vehicle is cornering to the right, there will be a tendency for the vehicle body to roll to the left, i. e. for the ride height at the right suspension unit 14 to increase and the ride height at the left hand suspension unit 16 to decrease by expansion and contraction of the respective gas springs 18. Assuming a simple transfer of load from right to left, the expansion of the right spring 18 is greater than the contraction of the left spring 18, and hence if the system operated passively the increase in ride height on the right of the vehicle would be greater than the decrease in ride height on the left of the vehicle. This increase in overall ride height is known as jacking.

The control unit 68 therefore operates the control valve 56 so as to connect the first working chamber 48 to the right suspension unit 14 and the second working chamber 50 to the left hand suspension unit 16. It also operates the actuation valve 60 so as to connect the actuation chamber 46 to the reservoir 66. Assuming the fluid pressure difference between the two suspension units 14,16 is not too great, the fact that the first working chamber 48 has a larger cross section than the second working chamber 50 means that the piston 44 will tend to be pushed away from its rest position, i. e. to the right in Figure 1, increasing the length of the first working chamber 48 and decreasing the length of the second working chamber be equal amounts. This will increase the volume of hydraulic fluid in the left suspension unit 16, on the outside of the corner, and decrease the volume of fluid in the right hand suspension unit 14, on the inside of the corner. Because of the unequal areas of the working chambers 48,50 the amount of fluid allowed to escape from the right suspension unit 14 is greater than the amount introduced into the left suspension unit 16. The total volume of fluid in the two suspension units 14,16 is therefore reduced thereby tending to reduce the total height of the front end of the vehicle. This counteracts the jacking tendency resulting from the differential volume changes in the gas springs 18. The actuation valve 60 is pulsed open and closed so as to maintain the desired degree of roll of the vehicle. This may be zero, but it is generally preferable to allow a small degree of roll so that the driver gets some feedback of the cornering acceleration.

As the lateral acceleration decreases again, e. g. when the vehicle is coming out of a corner, the actuator piston 44 needs to be moved back towards the original rest position so as to return the fluid volumes in the suspension units 14,16 towards their original, equal, levels. This is done by operating the actuation valve 60 so as to connect the actuating chamber 46 to the accumulator 64 and pump 62.

Again the valve 60 is pulsed so that the relative volumes of the two working chambers is kept at the desired level to control the roll aspect of the vehicle.

Referring to Figure 2, in a second embodiment of the invention there are two control actuators 140 each operating between a respective pair of diagonally opposite suspension units of the vehicle, i. e. one 140a between the front right and rear left suspension units 114,115 and one-140b between the front left and rear right suspension units 116,117. Each of the suspension units 114,115,116,117 is a hydropneumatic suspension unit as described above and shown in Figure 1, and each of the control actuators 140 is the same as that described above and shown in Figure 1. It will be appreciated that this arrangement enables control of both roll and of the vehicle, and pitch, i. e. rotation about the horizontal transverse axis T T.

In a third embodiment, which is not shown in the drawings, the suspension units of the front two wheels are interconnected as in the first embodiment, and those of the rear two wheels are also interconnected by a similar arrangement.

This means that the two control actuators can be connected up in the same sense to simultaneously control roll of the vehicle, or in the opposite sense so as to control cross articulation, that is when one diagonally opposite pair of wheels moves upwards relative to the body and the other diagonally opposite pair of wheels moves downwards.

It will be appreciated that, though a hydraulic actuation system for the pistons 44 is described above, any form of actuation could be used. In particular electric motors could be used which would give more even control and could be used to power the piston in either direction. Thus the fluid pressure in the suspension units would not have to be relied upon to move the pistons in one direction.

Claims

CLAIMS 1. A vehicle suspension system for controlling movement of two vehicle wheels relative to a vehicle body, the system comprising two suspension assemblies each associated with one of said wheels and each comprising a spring and a hydraulic actuator acting in series to control movement of the respective wheel relative to the body, and control means arranged to detect a condition when the wheels will tend to move in opposite directions relative to the body, and in response to-cause hydraulic fluid to flow out of a first one of the hydraulic actuators and into a second one thereby to counteract the tendency to move, wherein the control means is arranged to cause more fluid to flow out of the first hydraulic assembly than to flow into the second thereby reducing the total volume of fluid in the two actuators.
2. A system according to claim 1 wherein the control means includes cylinder divided into two working chambers by a double acting piston, each of the chambers being connectable to a respective one of the actuators and the piston being movable in the cylinder such that fluid flows out of the first actuator into the second, wherein the working cross sectional area of a first side of the piston is greater than that on a second side.
3. A system according to claim 2 wherein the control means further includes valve means arranged to connect a first one of the working chambers on said first side of the piston to the first actuator and a second working chamber on the second side of the piston to the second actuator.
4. A system according to claim 3 wherein the valve means is switchable so as to connect each working chamber to either actuator depending on which of the wheels is tending to move in which direction relative to the body.
5. A system according to claim 3 or claim 4 wherein the piston has an inoperative position at one end of its travel where the volume of the first working chamber is at a minimum and is movable so as to increase the volume of the first chamber on detection of said condition.
6. A system according to any one of claims 2 to 5 wherein the control means further comprises driving means arranged to drive the piston along the cylinder so as to vary the volumes of the chambers.
7. A system according to claim 6 wherein the driving means is hydraulically actuated.
8. A system according to claim 7 wherein the cylinder defines a further chamber and the piston is formed as part of a movable member which has a face defining one wall of said further chamber and the control means is arranged to control the flow of hydraulic fluid into and out of said further chamber to control the movement of the piston.
9. A system according to any foregoing claim wherein the wheels are on opposite sides of a longitudinal axis of the vehicle and the system is arranged to control roll of the vehicle body about said longitudinal axis.
10. A system according to any foregoing claim wherein the wheels are on opposite sides of a transverse axis of the vehicle and the system is arranged to control pitch of the vehicle body about said transverse axis.
11. A system according to any foregoing claim wherein said two suspension assemblies are each associated with one of a first pair of diagonally opposed wheels of the vehicle and the system comprises two further such suspension assemblies each associated with one of a further pair of diagonally opposed wheels and a further control means arranged to control the flow of fluid between the hydraulic assemblies of the two further suspension assemblies, the control means being operable to control roll of the body about a longitudinal axis and pitch of the body about a transverse axis.
12. A system according to any one of claims 1 to 10 wherein said two suspension assemblies are each associated with a front pair of wheels of the vehicle and the system comprises two further such suspension assemblies each associated with one of a back pair of wheels and a further control means arranged to control the flow of fluid between the hydraulic assemblies of the two further suspension assemblies, the control means being operable to control roll of the body about a longitudinal axis and cross articulation of the wheels relative to the body.
13. A vehicle suspension system substantially as hereinbefore described with reference to the accompanying drawings.